The invention relates generally to transfer case clutch configurations and more specifically to a transfer case having a double overrunning clutch and torque limited centrifugal clutch in mechanical parallel.
Motor vehicle four wheel drive systems have become increasingly sophisticated during the last decade. Such sophistication has been accompanied by improved understanding of vehicle dynamics and the more fundamental requirements of such systems. Better understanding of the fundamentals of vehicle dynamics has also encouraged a reexamination of less sophisticated mechanical systems.
For example, it was once proposed to provide four wheel drive from a primary, typically rear, drive line to a secondary or front drive line through a one-way or overrunning clutch which would engage and transfer torque from the rear drive line to the front drive line whenever the rear drive line attempted to overspeed the front drive line, typically because of a loss of traction of the rear drive wheels. However, due to the characteristics of one-way clutches, the system was incapable of providing four wheel drive in reverse. This deficiency was problematic in that the drive system would provide only two wheel drive when, for example, attempting to back out of deep snow or mud or while in reverse when attempting to rock the vehicle.
Also, at speed, a transfer case utilizing a one-way clutch could produce vehicle instability if the foot was lifted off of the throttle and the engine compression braking, which would slow only the rear wheels, created sufficient drag to break traction on these drive wheels.
It is known that the higher the engine speed, the more compression braking the engine creates in an off throttle condition. Since vehicle speed is proportional to engine speed, engine compression braking is also proportional to vehicle speed. The need for torque delivery to the front drive line to overcome the possible instability generating effects of engine braking at the rear wheels is also directly proportional of vehicle speed. Since the speed of the drive shafts of a vehicle also obviously varies with vehicle speed, a device which achieves cross drive line torque transfer, i.e., from the front drive line to the rear drive line in proportion to speed, will compensate for the effects of engine braking and improve vehicle performance and stability.
Another operating issue relating to both one-way and double overrunning clutches involves their rate of engagement. While they generally operate rapidly and such rapid operation is desirable, it would be preferable if the rate of engagement could be controlled. With standard mechanical overrunning clutches, this is not achievable. Accordingly, alternate approaches involving the combination of overrunning clutches with other torque transfer devices has been examined. The present invention is directed to such a device having parallel torque paths.